Field of the Invention
The present invention relates to a honeycomb structure. More specifically, the invention relates to a honeycomb structure capable of preventing a breakage caused by thermal stress while having a low pressure loss at an initial timing and a timing in which a large amount of particulate matter such as soot or ash is accumulated and accumulating a large amount of particulate matter such as soot or ash.
Description of the Related Art
Hitherto, a converter obtained by coating a three-way catalyst on a honeycomb structure has been used to remove CO, HC, and NOx (removal of toxic elements) in an exhaust gas of a gasoline engine. Then, it is most effective to use the converter in order to remove toxic elements. However, a direct fuel injection type gasoline engine is used from the viewpoint of reducing the amount of CO2. The direct fuel injection type gasoline engine has a problem in which exhaust particulates are emitted. For this reason, a sufficient effect cannot be obtained by the existing converter.
Further, a diesel engine has an excellent thermal efficiency compared with the gasoline engine. Thus, there is an advantage that the diesel engine can be appropriately used as an automobile engine that reduces the amount of CO2 as a countermeasure for global warming. Meanwhile, the diesel engine has a problem in which particulates are generated due to a diffusing combustion. Since the particulates cause a cancer, it is essential to prevent the particulates from being emitted to the atmosphere. For that reason, there is currently a strict regulation that regulates the number of particulates in addition to the regulation of the particulate emission amount based on the weight of the related art.
Here, there is a limitation in the reduction of the particulate emission amount caused by the improvement in combustion. Nowadays, only one effective method is to install a filter in an exhaust system. As the filter, a wall flow type filter that is designed to cause a gas to pass through a porous partition wall is effective. That is, there is proposed a honeycomb structure which includes a plurality of cells as a most effective measure at present. Here, the cells of the honeycomb structure capable of using the surface of the partition wall as a filtration area are formed so that an inflow end face and an outflow end face are alternately plugged and an exhaust gas flows through the partition wall so as to trap particulates. Here, the honeycomb structure is most effective currently. However, the honeycomb structure (the wall flow type filter) has a problem in which a filtration rate needs to be decreased in order to reduce the pressure loss within an allowable range.
For that reason, such a wall flow type filter is used to reduce a pressure loss when a large amount of particulate matter (PM) such as soot or ash is accumulated (in a PM accumulation state). Here, a problem arises in that the initial pressure loss increases when the filtration area or the open frontal area of the cell at the side of the inflow end face is increased.
Further, the wall flow type filter generates locally excessive heat when the accumulated soot is burned. In that case, a crack is generated due to the generated heat.
To solve such a problem, there is proposed a filter that suppresses both an initial pressure loss and a pressure loss in a PM accumulation state, prevents excessive heat locally generated inside the filter during the combustion of soot, and reduces a crack caused by thermal stress (see Patent Document 1).
[Patent Document 1] JP-A-2014-200741